The discriminative effects of narcotic agonists and antagonists will be systematically evaluated in rats trained to discriminate between saline and morphine and in squirrel monkeys trained to discriminate between saline and morphine and between saline and cyclazocine. Principal objectives will include: 1) an assessment of stimulus generalization as a function of training dose, 2) an analysis of discriminative properties in terms of a three-receptor model (Martin et al., J. Pharmacol. Exp. Ther. 197:517-532, 1976), and 3) a determination of the feasibility of training animals to discriminate between the morphine-dependent state and the state of antagonist-precipitated withdrawal. A study of the effect of narcotic agonists and antagonists on behavior maintained in the rat by intracranial electrical self-stimulation of the brain will continue. The effects of morphine and naltrexone on a lever-press response for electrical stimulation of the medial forebrain bundle will be compared in nontolerant, in morphine-tolerant/dependent, and in post-morphine-tolerant/dependent animals.